This invention relates generally to reset circuits and particularly to a reset circuit that operates reliably down to two volts DC.
There are many prior art circuits available for establishing and maintaining a reset condition in a voltage sensitive utilization device, in the event the supply voltage drops to a level where functioning of the utilization device may become unreliable. While the circuit of the invention is particularly useful with microprocessors, it will find application in any circuit where logic gates are being controlled and where, under low voltage supply conditions, their operation may become unstable or unreliable.
Many prior art circuits utilize zener diodes to establish a reference potential at which a reset signal is developed or a reset condition is otherwise established. Two difficulties are encountered, the first being that a zener diode is generally not operable below three volts DC, and secondly, that its voltage breakdown characteristic has what is called a "soft knee" which makes for somewhat unstable operation in that area. A reset arrangement for a microprocessor circuit, for example, must operate at a trigger voltage level that is high enough to insure that the microprocessor is still operating reliably. The reset condition or signal must then be maintained until the operating potential is again at a sufficient level to assure that the microprocessor will operate reliably. For example, a conventional "5 volt" microprocessor becomes unstable if the voltage drops to 3 volts. A reset circuit should, therefore, desirably be initiated when the voltage drops to a trigger level of about 3.5 volts and be maintained for a voltage down to 2 or 2.5 volts. It is also highly desirable that the reset circuit maintain the microprocessor in a reset condition until the supply voltage rises to a level at which the microprocessor will operate reliably, preferably a level that is above the trigger level. None of the prior art circuits fulfill the above conditions.
Further, it is also desirable that the reset circuit respond very quickly so that momentary voltage drops in voltage will not result in erroneous data or information from the microprocessor controlled circuit or other utilization device. It is also further desirable that such a circuit be fairly simple and economical. The present invention circuit satisfies all of these needs.